Television system



May 19, 1936.

TELEVISION BROADCAST CHANNELS FREQUENCY INTERMEDIATE FRE UENCY CHARACTERISTICS ft/21 AUDIO AUDIO FRE'QL DE'T. AMP.

? PICTURE PICTURE PICTURE INT-FREQ." DE'T'. AMP.

SOUND AND PICTUREZ SUPERHE'I'ERODYNE RECEIVER Wendell L. Carl son Fig. 3,

BY M

70 SPEAKER r0 CH THODE RA Y 'ruaz INVENTOR:

Patented May 19, 1936 UNITED STATES TELEVISION SYSTEM Wendell L. Carlson,

Haddonfield, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application January 31, 1934, Serial No. 709,072

9 Claims.

My invention relates to television systems and more particularly to systems whereby signals representing a picture and signals representing sound may simultaneously be transmitted and received without mutual interference.

As is well known to those skilled in the art, for the successful transmission of television pictures having good definition, it is necessary to allocate a picture frequency band of at least one megacycle wide. It is also a known fact that the number of channels available at the higher carrier frequencies necessary for successful television broadcasting is limited. For these reasons it becomes highly desirable to provide means whereby the available frequency bands may more economically be utilized than by utilizing entirely separate frequency-channel assignments for sound and picture signals.

It is, accordingly, an object of my invention to provide a television system that shall enable more economical use of available frequency bands for broadcasting.

Another object of my invention is to provide, in a system of the type described, means whereby a television receiver, which of necessity must include extremely broad picture circuits, may be sharply tuned through observance of the tuning of the said receiver to receive the sound signals.

Another object of my invention is to provide, in a system of. the type described, means whereby the simultaneous reception of separate sound and picture signals from a given transmitting station may be had without interference between each other or between such signals from other television transmitters in adjacent or even in widely separated bands.

Another object of my invention is to provide, in a system of the type described, means for simulating the effect of single side band and carrier transmission whereby economical use may be made of available television channels without resorting to complicated apparatus usually required for side-band suppression at a transmitter.

A still further and more specific object of my invention is to provide means whereby sound and picture signals may be transmitted and satisfactorily received on a plurality of channels, wherein the total frequency range required is determined by the picture frequency side band requirements and wherein no additional frequency range is required to accommodate the sound transmission.

The foregoing objects and other objects ancillary thereto I propose to accomplish by making use of the fact that if the total energy in the picture transmission channels is kept substantially equivalent to the total energy in the accompanying sound channels, the average distribution of the side band energy in the picture channels within a finite frequency band is vastly less than the average distribution of the side band energy in the same finite frequency band of the sound channels, which channels, as is well known, need be only 10 or 20 k. 0. wide. Furthermore, I make use of the fact, which I have discovered, 10 that the sound transmission may advantageously be located at a point in the next adjacent picture channel, i. e. within the picture side band frequency band which is adjacent to the picture transmission accompanying the said sound transmission.

Furthermore, in accordance with my invention I provide a superheterodyne receiver of the unicontrol type and so select the constants of the said receiver that the picture signal amplification portion thereof passes efficiently only one set ofside-b'an'd frequencies of the picture signal, with carrier, and the sound signal amplification portion thereof passes efficiently the complete sound signal. Byreason of this choice of constants and having in mind the low average energy in the side-bands accompanying the picture carrier in the next adjacent channel, at the frequency of the aforesaid sound channel, I am thus enabled to receive the sound signals 8.0- companying the picture with but slight if any interference from the picture on the adjacent channel.

The novel features that I consider characteristic of my invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and its method of operation, together with additional objects and advantages thereof, will best be understood from the following description of a specific 40 embodiment, when read in connection with the accompanying drawing, in which:

Fig. 1 is a diagram exemplifying the relative disposition of picture and sound channels according to my invention;

Fig. 2 is a diagram exemplifying the manner in which I accomplish simultaneous reception of picture and sound signals; and

Fig. 3 is a conventionalized diagram illustrating a receiver constructed according to my invention. Referring now particularly to Fig. 1 of the drawing, it will be noted that the television broadcast frequencies are indicated therein by the designations 49 m. c., 50 m. c., 51 m. c., 52 m. c., 53 m. 0., 54 m. c. and 55 m. c. Thisis merely 55 indicative of the fact that each of the picture carriers at 52, 54, etc. are spaced 2 m. c. apart and that the sound carriers are located at 51.5, 53.5 etc. It will be noted that in the arrangement of Fig. 1 each picture carrier and one 1 m. 0. side band is clear but that the other picture side band shares part of its frequency allocation with the sound carrier and its k. c. side bands. It is, of course, to be understood that I am not limited to the above frequency widths exemplified in Fig. 1 of the drawing nor to the number of station channels disclosed.

Referring now to Fig. 3 of the drawing, in order to satisfactorily receive pictures on channel A and accompanying sound on channel B of Fig. 1, I provide a superheterodyne radio receiver having a common radio frequency amplification stage or circuits I, acommon first detector 3, a common oscillator 5, and a plurality of individual channels I and 9 allocated respectively to picture and sound. Obviously, the picture signals may be applied to any desirable reproducing device, such as the now well known cathode ray tube, or to any other desired device and the audio signals may be applied to a loudspeaker of any satisfactory type. Since these specific elements, picture receiving device and loudspeaker, form no part of my present invention no necessity is seen for illustrating them.

In accordance with my invention the radio frequency tuned circuits of the receiver are bandpass in character. In other words, they must, for satisfactory reception, cover a band of frequencies at least 1 m. 0. wide plus an additional band 505 k. c. wide. That is to say, the tuning of these stages must be such that they efiiciently amplify a band of frequencies having a total width of 1,505 k. 0. so as to pass the picture carrier and one side-band thereof as well as the sound carrier and its side-bands, to the first detector. This characteristic can, of course, be obtained, as is well known to those skilled in the art, through the use of coupled tuned circuits provided, if necessary, with damping resistors, or the like, and the specific circuits need not be illustrated.

In the specific example chosen for purposes of illustration the picture carrier frequency A is 52 m. c. and the sound carrier frequency is 53 m. c. According to my invention, and as exemplified by Fig. 2 of the drawing, I propose to so design the intermediate frequency amplifier in the picture channel that it shall pass only frequencies corresponding to the upper picture side-bands and shall attenuate frequencies lying below the picture carrier frequency and frequencies lying in the lower side-band range of the next higher frequency channel. To this end, I provide in the intermediate frequency amplifier handling the converted picture signals one or more band-pass filters or a plurality of coupled tuned circuits designed to pass a band of frequencies corresponding to the upper side-band frequencies and carrier which, in the example chosen, lies between 9.55 m. c. to 8.55 m. c. To obtain these new converted frequencies, I propose to utilize a local oscillation frequency of 61.55 m. c.

The beat frequencies produced through interaction between the local oscillator and the incoming carrier, within the range shown in Fig. 2 of the drawing are effectively amplified, but, as will be noted, frequencies above and below the limits of the range are rapidly attenuated.

It will also be noted that the difference between the local oscillator frequency and the carrier frequency allocated to the sound channel provides an intermediate frequency of 8.05 k. c. to which frequency the intermediate frequency amplifying stages of the sound channel are tuned. Attention is further directed to the fact, as exemplified by Fig. 2 of the drawing that the attenuation of the intermediate frequency corresponding to the sound is very rapid at frequencies above and below the side-band range necessary and for that reason, the two sets of intermediate frequencies, namely picture and sound, may easily be separated in the receiver and allocated to their proper channels.

From the foregoing, it might be assumed that my invention is limited to the specific intermediate and oscillator frequencies chosen. Such is not the case, however, since other desirable intermediate frequency pairs may be utilized. For example, to avoid radiation interference from the local oscillator to nearby receivers and reduce image frequency interference from transmissions on frequencies equally spaced on the opposite side of the local oscillator, I propose to employ one of the following groups of intermediate frequencies so as to stagger the interfering frequency bands in reference to other station channels. The preferred intermediate frequencies for a system with station frequency assignments as arbitrarily chosen and referred to hereinbefore are:

Group Sound Picture 1 6.05 m 0.515 to 30k. 0. 6.55 to 7.65

2 8.05 m c.;i;5 to 30 k. c. 8.55 to 9.65

3 10.05 m c.:i:5 to 30 k. c. l0.55toll.65

By Way of further explanation of the manner in which my improved system operates, attention is called to the following facts:

Referring to Fig. l; assume that picture carrier A at 52 m. c. with double side band transmission over a band of 2,000 k. c. is accompanied with sound transmission B at 53.5 m. c. with side bands covering a total of k. c. C is the adjacent channel sound carrier at 51.5 m. c. and D is the adjacent channel picture carrier at 54 m. c. The double channel superheterodyne receiver. Fig. 3, separates the tWo signals by means of the intermediate frequency amplifier selectivity represented by the characteristics in Fig. 2.

Interference from adjacent sound signals C and B on picture carrier A and associated side bands is avoided by selectivity characteristic E in Fig. 2 which restricts the picture reception mainly to the carrier and one clear side band. It should be understood that the transmissions represented in Fig. 1 have been converted to intermediate frequencies when referring to Fig. 2, but that the channel-frequency separation is not changed.

The adjacent picture interference from the side band of carrier D in Fig. 1 on sound transmission B through sound amplifier characteristic F in Fig. 2 is avoided partly by the difference in means amplitude of picture and sound frequencies within the sound frequency band.

The sound and picture carrier frequencies A, B, C and D in Fig. 1 have twice the amplitude of any of their side band frequencies when modulated 100%. At a given instant the amplitude of a particular picture side band frequency may equal the amplitude of a particular sound side band frequency. But over an appreciable interval of time the average amplitude of picture and sound side band frequencies within the same finite frequency band will be approximately inversely proportional to the side band frequency ranges of each transmission.

Assume a carrier frequency modulated and that the sound modulation frequency is changing at a uniform rate between 0 and 5 k. c. producing side bands within a 10 k. c. frequency band, i. e. carrier :5 k. c. The amplitude in each side band over a period of time will be VT, where V=volts at one instant and T=time. Assume in another case that the picture modulation frequency is changing at a uniform rate between 0 and 1,000 k. c. producing side bands within a 2,000 k. 0. frequency band, i. e. carrier .:1,000 k. c. The energy in each side band over the same period of time will be the same for the second case as for the first case but will be spread out over 200 times greater frequency range.

The side band average amplitude within any portion of the total frequency range in one side band will, over a period of time, be

VTX

53.5 m. 0. i5 k. 0. will be VT10 E 1000100 The average sound side band amplitude over a period of time, therefore, is 200 times as great as the average interfering picture side band amplitude within the same frequency band.

It should be clear from the above analysis that even though the instantaneous side band interference from the picture transmission D may equal 1/2 the double side band transmission in sound signal B, that the average interference will be 1/200 when assuming equal field strength reception from transmissions B and D and equal percentage modulations of both carriers. It should also be clear that this favorable interference ratio is made possible by the great difference in side band frequency ranges required for the two transmissions.

When dealing with signals of widely different character it is difiicult to evaluate the interference in numerical terms proportional to observed annoyance. The interference Which will be observed at the loudspeaker from the intermittent picture side band frequencies within the sound frequency band B will be less than the maximum judged by intermittent peak amplitudes, and more than the minimum judged by the average amplitude over a period of time. Observations on practical tests, however, indicate that the mean value of interference is approximately between these extremes, i. e. between 1/2 and 1/200 or about l/10 or 1/20.

It is contemplated that the adjacent channel side band field strength within the maximum service range of the desired station will be attenuated about ten times. This will be accomplished either by proper geographical allocation of adjacent channel stations or by partial suppression of the interfering picture side band at the transmitter.

It is estimated if the interference on the desired sound channel were another sound transmission covering the same frequency band as the desired sound transmission that the field strength 1 ratio required would be about 1000/1 and if the carrier were removed from the interfering transmission, the ratio required would be about 100/ 1. In other words, heretofore the field strength ratio necessary to avoid interference between transmissions covering the same radio frequency band has been between the ratios 100/1 and 1000/1. In my improved system with transmission covering widely dilferent frequency bands, the field strength ratio necessary to avoid interference may only be in the order of 10/1. Part of the interference problem is solved by field strength ratios as heretofore utilized in the radio art and part by the ratios of side band frequency spread.

To simplify the analysis in this discussion, it has been assumed that the modulation for both picture and sound is held constant at 100%. Also, it has been assumed that there will be uniform distribution of side band energy within all f finite frequency bands of the legitimate side band frequency ranges of each signal. These assumptions are considered justified for purposes of analysis on the ground that practical experiments with sound and picture transmissions under conditions of random variation of percent modulation and frequency distribution have supported the theory advanced herein.

For practical operation of the system, it is essential to have the sound transmission within the frequency band of the adjacent picture side band instead of within the frequency band of its accompanying picture side band. Otherwise, discrimination in geographic location could not be utilized toproduce a 10 to 1 field strength ratio between sound and picture transmissions. If the sound transmission were located within the frequency band of its accompanying picture side band, it would be necessary to rely entirely on suppressing one picture side band at the transmitter, which is very difficult to accomplish, and to keep in proper service adjustment. For practical commercial reasons, therefore, it is .advisable to inaugurate a television system which does not depend for successful operation on suppressed side band transmission but which can utilize suppressed side band transmission to advantage as the art progresses and without obsoleting old receivers.

From a consideration of the foregoing it will be obvious that I have provided an improved television transmission and reception system whereby mediate channel. If, therefore, an attempt is made to tune the system merely through observation of the received picture, this process would be extremely difficult since the only indication of correct tuning would be the definition of the picture and not its brightness. On the other hand, the ear can detect the correct position of tuning due to the relatively sharply tuned sound intermediate frequency channel.

It has been determined that limiting the intermediate picture amplifier to a band width for carrier and one side band, and the radio frequency tuned circuits to the sound signal, the picture carrier and one side band has the advantage of increasing the amplification gain per stage. The amplification is inversely proportional to the band width. For practical reasons, it is not advisable to operate the intermediate carrier frequency at the edge of the band pass characteristic but rather to operate it so that both side bands of the lowest picture modulation frequencies are reproduced. This arrangement has been found to cause no objectionable distortion when the picture frequency amplifier corresponding to the audio amplifier on the sound channel is properly compensated.

Many other advantages accruing through use of my improved system will be apparent to those skilled in the art as well as many obvious modifications thereof. My invention, therefore, is not to be restricted except insofar as necessitated by prior art and by the spirit of the appended claims.

I claim as my invention:

1. In a system in which a plurality of modulated picture carriers are transmitted simultaneously, the method of simultaneously transmitting pictures and accompanying sound which comprises transmitting the picture on a given carrier frequency and transmitting the sound on a carrier frequency lying substantially midway of one of the side band frequency ranges of the next adjacent picture carrier frequency.

2. The method of simultaneously transmitting pictures and accompanying sound in a plurality of picture channels wherein the picture carrier frequency of each channel is separated from the picture carrier frequency of the next adjacent channel by a frequency range equivalent to twice the highest modulation frequency required for satisfactory picture transmission, which comprises transmitting the picture on one of the said picture carrier frequencies and transmitting the accompanying sound on a carrier frequency midway of the side-band range of the next adjacent side band of another of said picture channels.

3. In a system in which a plurality of modulated picture and sound carriers are transmitted simultaneously and in which the sound carrier accompanying one picture carrier lies substantially midway of one of the side band frequency ranges of the next adjacent picture carrier, the method of simultaneously receiving sound and picture signals which comprises simultaneously heterodyning both carriers by locally generated oscillations, allocating the beat frequencies produced to a plurality of intermediate frequency amplifying channels and broadly tuning one of said channels to pass only intermediate frequencies corresponding to the side-band of the picture carrier frequency which is nearer said sound carrier and sharply tuning another of said channels to the sound-carrier frequency.

4. In a system in which a plurality of modulated picture and sound carriers are transmitted simultaneously and in which the sound carrier accompanying one picture carrier lies substantially midway of one of the side band frequency ranges of the next adjacent picture carrier, a combined picture and sound receiver comprising a broadly tunable radio frequency amplifier portion, a first detector, a local oscillator, a channel for amplifying beat frequencies corresponding to picture modulation, a channel for amplifying beat frequencies corresponding to sound modulation and means for simultaneously controlling the tuning of the radio frequency amplifier portion and the local oscillator, the receiver being further characterized in that the picture frequency amplifying channel is broadly tuned to pass beat frequencies corresponding to the side band of the picture carrier nearer said sound beat frequencies and to reject beat frequencies corresponding to the other side band of the picture carrier, and the sound frequency amplifying channel is sharply tuned to pass only the beat frequency within the range corresponding to the sound carrier and its side bands.

5. The method of simultaneously transmitting intelligence on a plurality of picture carrier waves and on a plurality of sound carrier waves which comprises transmitting the picture intelligence on picture carrier waves which are spaced by a frequency difference equal to twice the highest modulation frequency and transmitting the remainder of the intelligence on sound carrier waves, each sound carrier wave being spaced from its corresponding picture carrier wave by a frequency difference corresponding to one and one-half times the highest modulation frequency impressed on the said picture carrier wave.

6. The invention set forth in claim 4, additionally characterized in that the picture frequency amplifying channel is tuned to pass the beat frequencies corresponding to a picture carrier and one accompanying side-band and the sound amplifying channel is tuned to pass a beat frequency corresponding to a sound-carrier of a frequency differing from the said picture carrier frequency by a range equal to one and one-half times the highest modulation frequency applied to the said picture carrier.

'7. The method of transmitting intelligence which comprises applying double side-band modulation to a plurality of picture carrier waves and to a plurality of sound carrier waves, the spacing of said picture carrier waves being such that an upper side band and a lower side band of next adjacent picture carrier Waves are substantially contiguous in the frequency spectrum, and the spacing of a picture carrier wave and its accompanying sound carrier wave being such that said sound carrier wave lies within the next adjacent picture side band, transmitting said waves to a receiving station and thereat selecting and amplifying only the side-band of one of said picture carrier waves which is the nearer to its accompanying sound carrier wave while selecting and amplifying both side bands of said lastmentioned sound carrier wave.

8. The method set forth in claim 7 characterized in that the total side-band range of one of said picture carrier waves is two hundred times the total side-band range of the accompanying sound carrier wave, and that the sound carrier wave is spaced, in the frequency spectrum, from its accompanying picture carrier wave by a nonintegral multiple of the highest modulation frequency of the first carrier wave.

9. The method set forth in claim 7 wherein picture frequencies are utilized for the modulation of one of the carrier waves and sound-frequencies are utilized for modulation of the other carrier wave and further characterized in that the carrier wave which is sound-modulated is spaced, in the frequency spectrum, from the carrier wave which is picture-modulated, by an amount equal to a non-integral multiple of the highest picture modulation frequency.

WENDELL L. CARLSON. 

